A conventional antenna device contains a plurality of antenna elements, and a phase-shifter and an amplifier are connected just below each of the antenna elements. Proper control of phase-shifters and amplifiers allows an antenna device to have desired directivity. Such a conventional structure is disclosed, for example, in Japanese Patent Unexamined Publication No. 2001-024431. FIG. 35 and FIG. 36 show an antenna device in which the directivity can be controlled by a simple circuit structure.
The antenna device shown in FIGS. 35 and 36 contains radiating element 101, and at least one parasitic element 102 so as to keep an interval from radiating element 101. Radiating element 101 receives a radio-wave signal, whereas parasitic element 102 does not receive the radio-wave signal. Variable reactance element 103 is connected to parasitic element 102. Reactance value Xn of variable reactance element 103 is changed according to data obtained by transmitting/receiving circuit 104, whereby directivity of the antenna device can be controlled.
Such structured conventional antenna device can control directivity with high accuracy. The structure, however, due to the needs for a plurality of radiation elements 101 and a plurality of parasitic elements 102, has a difficulty in reducing the size of the device. Besides, a complicated controller is required to control a plurality of variable reactance elements 103.